A conventional seat belt system comprises a length of seat belt webbing connected at three-points to load-bearing parts of a vehicle. The webbing is arranged to have a lap portion that passes laterally across the hips of a seat occupant, and a torso portion that passes diagonally across the torso of the seat occupant from one hip to the opposite shoulder.
Typically one end of the webbing is attached to a sill anchor that is bolted to a load-bearing longitudinal structural member on one side of the vehicle seat, usually between the vehicle seat and an adjacent door. The lap and torso portions join at a buckle mechanism on the opposite side of the vehicle seat. The shoulder end of the webbing is attached to a seat belt retractor mounted to a load-bearing part of the vehicle, for example a side pillar or sill, or directly to a load-bearing seat component, optionally via a webbing guide.
The seat belt retractor increases comfort for the seat occupant restrained by the seat belt by allowing the webbing to pay out under relatively low loads to enable limited movement of the restrained seat occupant, for example to reach in-car entertainment controls or storage compartments. The seat belt retractor is biased to keep the webbing relatively taut about the seat occupant and a locking element locks the seat belt retractor against webbing payout in the event an acceleration sensor senses rapid acceleration or deceleration indicative of a vehicle crash.
The belt webbing is fastened to the buckle mechanism by a buckle tongue that is attached to the webbing such that the buckle tongue can slide along the belt webbing. The proportions of the belt webbing making up the lap and torso portions can easily be varied to suit the size of a seat occupant.
Conventional seat belt systems of this sort tend to be unsuitable for vehicle seat occupants of shorter than average stature, particularly for children, because the shoulder fastening point is fixed at or above the height of the back of the vehicle seat to accommodate an average size person. This is particularly so in seat belt systems for locations in a vehicle other than the front vehicle seats. The torso portion of the seat belt tends to be badly positioned for a child or short person and usually passes too close or adjacent to the child's neck. Because the child does not fit into the adult seat belt properly the child's shoulder can roll out of the seat belt during a crash effectively making the seat belt a two-point lap belt only. This is dangerous because the lap portion alone will then take more force in a crash and will be more likely to inflict injuries than when a torso belt section is combined. In addition there is a danger of the child sliding forward under the lap portion.